Oxidants such as hypochlorite, comprising Free Available Chlorine (FAC), are widely used, for example, for water treatment. Conveniently, these oxidants may be generated electrochemically, e.g. from aqueous feedstock electrolytes containing sodium chloride, i.e. common salt, or other salts, using On-Site Generation (OSG) systems. Oxidants, which may comprise various other species of oxidant, such as one or more of peroxodisulfate (peroxydisulfate or persulfate), hydrogen peroxide, chlorine dioxide or ozone, for example, have also been generated using such systems. Mixed oxidants, formed from chloride containing feedstocks, have been demonstrated to have advantages over chlorine or hypochlorite alone, when treating some waterborne microorganisms that are chlorine resistant, or for treating biofilms, which tend to be negatively charged and more resistant to hypochlorite ion (FAC) disinfection.
A problem with known OSG systems for generating oxidants and mixed oxidants is premature oxidation and/or degradation of the electrodes and other system components. This places significant limitations on the selection of electrode materials and on the long term reliability and performance of OSG systems used for generating oxidants. It is well known that OSG systems for generation of oxidants, including hypochlorite, are preferably operated at relatively low current density (typically significantly lower than 150 mA/cm2, e.g. 30-80 mA/cm2) and under constant polarity, to prevent or reduce premature failure of the anodes due to oxidation of the surface layers or underlying substrates.
Known prior art systems for chlorine and mixed oxidant generation typically utilize Dimensionally Stable Anodes (DSA) and stainless steel or titanium cathodes. DSA electrodes are available at reasonable cost and provide an operational lifetime of about a year when operated at low current density (i.e. typically 30 to 80 mA/cm2). Operation at higher current densities to increase the daily output of oxidant is difficult without significantly or unacceptably reducing the operational lifetime of the DSA anode.
Another particular issue for OSG systems for chlorine generation from salt is the build-up of scale, e.g. from calcium and magnesium impurities found in feedstocks, such as impure salts, and also in freshwater and seawater, particularly at higher operating temperatures. Seawater, which has a salt concentration of about 0.6 Molar, potentially offers an attractive low cost feedstock for a single pass system, i.e. where an effectively unlimited supply may be continuously pumped through the OSG system. However, impurities in low cost feedstocks, such as solar salt or seawater, tend to cause unacceptable levels of scaling or fouling of the electrodes (in particular the cathode) and other system components.
As described for example, in U.S. Pat. No. 4,578,160 to Asano et al., issued Mar. 25, 1986, entitled “Method for electrolyzing dilute caustic alkali aqueous solution by periodically reversing electrode polarities,” in a system using electrodes of iron, nickel, or their alloys, such as stainless steel, periodic reversal of the polarity of the anode and cathode diminishes oxidation of the anode and reduces scaling and deposits of impurities. Thus, this mode of operation increases the time between maintenance cycles, and reduces the operating voltage of the cell, which lowers the amount of electrical energy (i.e. power) required for operating the cell.
On the other hand, for OSG generation of oxidants, the DSA electrodes conventionally used for OSG of chlorine and other oxidants are preferably not operated under reverse polarity to avoid causing premature degradation of the electrode material. This is because DSA electrodes comprise conductive oxides, such as oxides of ruthenium or iridium. Thus, under reverse polarity the oxides tend to breakdown causing the electrode to disintegrate prematurely, which severely shortens the electrode lifetime and reliability. Conversely, stainless steel cathodes are susceptible to oxidation (rusting) if operated in reverse polarity. Thus, in DSA electrode systems, other methods must be used to remove scale, such as flushing or cleaning the system with concentrated hydrochloric acid, so as to avoid a costly and time consuming process of disassembly and cleaning the system. To avoid handling of concentrated acids, it is preferable to take measures to prevent scaling.
To avoid or reduce scaling, OSG systems using DSA electrodes cannot be operated with impure salt, i.e. feedstocks with more than 0.1% calcium and magnesium. In practical terms, this also excludes use of impure, low cost salt feedstocks, or use of hard water or seawater with impurities that cause scaling or fouling of the system. Purification or filtration of the feedstock is required and/or water softeners are usually required. These add very significantly to the capital costs and operating costs of such systems.
Another disadvantage of DSA electrodes is that the electrode lifetime has strong thermal dependence, and their lifetime is significantly reduced when used beyond an optimal temperature range.
It is known to use diamond as an electrode material for electrolysis of sodium chloride or other chloride containing feedstocks. In U.S. Pat. No. 6,767,447 to Uno et al., (Permelec) issued Jul. 27, 2004, entitled “Electrolytic cell for hydrogen peroxide production and process for producing hydrogen peroxide,” it is proposed that diamond based electrode surfaces have an increased electrode life and also provide enhanced electrochemical performance for certain applications, e.g. those requiring a higher rate of hydroxyl radical (OH radical) formation, from dilute alkali solutions. However, it is disclosed that chloride salt concentrations are maintained below 0.1M, to avoid significant production of chlorine.
The use of diamond anodes is also reported, for example, in United States Patent Publication No. 2007/0 170070 to Uno et al. (Permelec), dated Jul. 26, 2007, entitled “Electrolysis cell for synthesizing perchloric acid compound and method for electrolytically synthesizing perchloric acid compound” and references cited therein. This application discloses use of diamond electrodes to improve current efficiency using feedstocks comprising higher concentrations (0.1M or more) of chloride or chloric acid, but nevertheless it achieves very low current efficiencies, i.e. at most 20%.
On the other hand, diamond electrodes are significantly more expensive to manufacture (i.e. by a factor of ˜10-100 times) than conventional DSA anodes and stainless steel cathodes of similar active surface area. On that basis alone, there is little incentive or motivation for operators to replace conventional systems based on the incumbent technology using DSA electrodes with systems using more expensive diamond electrodes, when regular maintenance and operation at low current density can provide an operational lifetime of about a year or sometimes even more for DSA electrodes when operated at low current densities (e.g. <100 mA/cm2).
However, yet another issue is the output capacity of available OSG systems, i.e. the required daily output of chlorine, which may be several pounds of chlorine or mixed oxidant per day. To provide sufficient daily output of oxidant, the system must be relatively large, e.g. it may typically be a table sized unit, having a volume of several cubic feet. It requires multiple cells or many pairs of electrodes operating at only 30 mA/cm2 to provide sufficient active area to provide the desired daily output of oxidant under conventional operating conditions. Operation at low current density limits the rate of production of oxidant, and a FAC concentration of 6000 to 8000 ppm may require extended hours of operation. As mentioned above, while seawater is a potential cheap feedstock, in addition to issues of scaling from impurities, these systems have limited capacity to rapidly produce a high enough concentration of FAC or other oxidants from seawater. Electrolyte feedstocks with higher salt concentrations are required, and must use expensive purified salt or rely on water softeners to avoid scaling and fouling of the system.
As mentioned above, mixed oxidants that comprise FAC together with other oxidants such as hydroxyl, hydrogen peroxide, et al. have been demonstrated to be more effective in treating chlorine resistant microorganisms (Water Conditioning and Purification, “On-Site Mixed Oxidants Generate Benefits in Puerto Rico”, Carlos Gonzalez, September 2002, p. 62-65). Thus it is desirable to develop processes that enhance production of mixed oxidants comprising hypochlorite and/or other oxidant species that are more effective in destroying such microorganisms.
In summary, a need exists to increase the cost efficiency and reliability of systems and methods for OSG production of oxidants and to increase the rate of production of oxidants. It would also be desirable to provide more compact systems and/or lower maintenance systems, which can produce a daily output of several pounds of chlorine or other oxidants, including mixed oxidants, to enable more flexible deployment, in a wider range of applications.
Thus, there is a need for improved or alternative solutions which address one or more of these shortcomings of known systems and methods for OSG of oxidants and mixed oxidants.